1. Field of the Invention
This invention pertains to devices comprising nanowires, in particular, field emitter structures.
2. Discussion of the Related Art
Electron field emitters are useful for a variety of applications including microwave-amplifiers and flat-panel, field-emission displays.
Microwave vacuum tube devices, such as power amplifiers, are essential components of many modern microwave systems including telecommunications, radar, electronic warfare, and navigation systems. While semiconductor microwave amplifiers are available, they generally lack the power capabilities required by most microwave systems. Microwave tube amplifiers, in contrast, provide microwave energy at much higher power levels. The higher power levels of tube devices are the result of the fact that electrons travel at a much higher velocity in a vacuum than in a semiconductor. The higher speed permits use of larger structures with the same transit time. Larger structures, in turn, permit greater power levels.
Microwave tube devices typically operate by introducing a beam of electrons into a region where the beam interacts with an input signal, and then deriving an output signal from the modulated electron beam. See, e.g., A. W. Scott, Understanding Microwaves, Ch. 12, John Wiley and Sons (1993), the disclosure of which is hereby incorporated by reference. Microwave tube devices include traveling wave tubes, gridded tubes, klystons, cross-field amplifiers and gyrotrons. The usual source of electrons for microwave tube devices is a thermionic emission cathode, typically formed from tungsten cathodes, optionally coated with barium oxide or mixed with thorium oxide. The cathode is heated to a temperature around 1000xc2x0 C. to produce thermionic electron emission on the order of amperes per square centimeter.
The requisite heating of thermionic cathodes causes a number of problems. Cathode lifetime is limited because key constituents of the cathode, such as barium oxide, evaporate under the high operating temperatures, and when the barium is depleted, the cathode (and hence the tube) no longer perform. Many traveling wave tubes (TWTs), for example, have operating lives of less than a year. Also, the need to raise the cathode to the operating temperature causes emission delays of up to several minutes, which is not acceptable for most commercial applications. In addition, the high temperature operation generally requires a peripheral cooling system, such as a fan, thereby increasing the size of the overall device or system. It would therefore be desirable to develop microwave tube devices that do not require such high temperature operation, e.g., cold cathode devices.
Another promising application of field emitters is thin, matrix-addressable, flat panel displays. See, for example, Semiconductor International, December 1991, p.46; C. A Spindt et al., IEEE Transactions on Electron Devices, Vol. 38, 2355 (1991); I. Brodie and C. A. Spindt, Advances in Electronics and Electron Physics, edited by P. W. Hawkes, Vol. 83, pp. 1 (1992); and J. A Costellano, Handbook of Display Technology, Academic Press, 254 (1992); and U.S. Pat. Nos. 4,940,916; 5,129,850; 5,138,237 and 5,283,500, the disclosures of which are hereby incorporated by reference.
A variety of characteristics are known to be advantageous for cathode materials of field emission devices. The emission current is advantageously voltage controllable, with driver voltages in a range obtainable from xe2x80x9coff the shelfxe2x80x9d integrated circuits. For typical device dimensions (e.g. 1 xcexcm gate-to-cathode spacing), a cathode that emits at fields of 25 V/xcexcm or less is generally desirable for typical CMOS circuitry. The emitting current density is advantageously in the range of 1-10 mA/cm2 for flat panel display applications and greater than 100 mA/cm2 for microwave power amplifier applications. The emission characteristics are advantageously reproducible from one source to another and advantageously stable over a very long period of time (tens of thousands of hours). The emission fluctuations (noise) are advantageously small enough to avoid limiting device performance. The cathode is advantageously resistant to unwanted occurrences in the vacuum environment, such as ion bombardment, chemical reaction with residual gases, temperature extremes, and arcing. Finally, the cathode manufacturing is advantageously inexpensive, e.g. no highly critical processes and adaptable to a wide variety of applications.
Conventional field emission cathode materials are typically made of metal (such as Mo) or semiconductor material (such as Si) with tips of submicron size. While useful emission characteristics have been demonstrated for these materials, the control voltage required for emission is relatively high (around 100 V) because of their high work functions and insufficiently sharp tips. This high voltage operation increases the damaging instabilities due to ion bombardment and surface diffusion on the emitter tips and necessitates high power densities to be supplied from an external source to produce the required emission current density. The fabrication of uniform tips is difficult, tedious and expensive, especially over a large area. In addition, the vulnerability of these materials to conditions of a typical operating environment, e.g., ion bombardment, reaction with chemically active species, and temperature extremes, is of concern.
Carbon materials (diamond and carbon nanotubes) have recently emerged as potentially useful electron field emitters. Diamond offers advantages due to the negative or low electron affinity on its hydrogen-terminated surfaces, but the technological advances have been somewhat slow because of emission non-uniformity and the tendency for graphitization in diamond emitters at increased emission currents, e.g., above about 30 mA/cm2.
Carbon nanotubes feature a high aspect ratio ( greater than 1,000) and a small tip radii of curvature (xcx9c5-50 nm). These geometric characteristics, coupled with the high mechanical strength and chemical stability of the tubules, make carbon nanotubes attractive as electron field emitters. See, e.g., German patent No. 4,405,768; Rinzler et al., Science, Vol. 269, 1550 (1995); De Heer et al., Science, Vol. 270, 1179 (1995); Saito et al., Jpn. J. Appl. Phys., Vol. 37, L346 (1998); Wang et al., Appl. Phys. Lett., Vol. 70, 3308 (1997); Saito et al., Jpn. J. Appl. Phys., Vol. 36, L1340 (1997); and Wang et al., Appl. Phys. Lett., Vol. 72, 2912 (1998), the disclosures of which are hereby incorporated by reference. However, carbon nanotubes are typically available in the form of needle-like or spaghetti-like powders which are not easily or conveniently incorporated into a field emitter device structure. And due to this random configuration, the electron emission capabilities are not fully utilized. In addition, adherence of thin film nanotubes to a conductive substrate is problematic because the nanotube material is usually free of dangling bonds and high energy sites, making chemical bonding to the substrate difficult. Other types of nano-scale wires with small diameters also exist. Semiconductor or metallic nanowires of silicon or germanium, for example, are capable of being fabricated by a number of different methods including laser processing, vapor-liquid approach or CVD deposition. See, e.g., A. M. Morales and C. M. Lieber, Science, Vol. 279,:208 (1998); A. J. Read et al., Phys. Rev. Lett., Vol. 69, 1232 (1992); J. Westwater et al., J. Vac. Sci. Technol., Vol. B15, 554 (1997); and T. J. Trentler et al., Science, Vol. 270, 1791 (1995). However, whether such nanowires are capable of successful incorporation in field emission structures is not clear. (As used herein, nanowires indicates wires having average diameters ranging from about 0.5 nm to about 50 nm and aspect ratios of about 100 to about 10,000.)
Fabrication techniques for obtaining useful nanowire emission structures are therefore desired.
The invention provides an improved process for fabricating emitter structures from nanowires. Specifically, the nanowires are coated with a magnetic material to allow useful alignment of the wires in the emitter array, and techniques are utilized to provide desirable protrusion of the aligned nanowires in the final structure. (Aligned indicates that the average deviation from perfect alignment normal to the supporting surface, at the point on the surface from which the nanotube protrudes, is less than 45xc2x0, as determined, for example, using high-resolution scanning electron microscopy.) In one embodiment, nanowires at least partially coated by a magnetic material are provided, the nanowires having an average length of about 0.1 xcexcm to about 10,000 xcexcm. The nanowires are mixed in a liquid medium, and a magnetic field is applied to align the nanowires. The liquid medium is provided with a precursor material capable of consolidation into a solid matrix, e.g., conductive particles or a metal""salt, the matrix securing the nanowires in an aligned orientation. A portion of the aligned nanowires are exposed, e.g., by etching a surface portion of the matrix material, to provide an average protrusion of at least twice the average diameter of the nanowires. Advantageously, a substrate is provided during the alignment step such that one end of each nanowire is pulled to and against the substrate by the magnetic field, thereby orienting the tips of the nanowires substantially along a single plane (see, e.g., FIGS. 1E and 1F).
The resultant structure offers several advantageous properties. The protrusion offers improved field emission; the intermixing of nanowires with a matrix material to form a composite provides a relatively high density of nanowire tips compared to other formation techniques; nanowires of relatively uniform height, which increases the number of nanowires participating in emission; and the composite material offers relatively stable electrical and mechanical contact between the emitters and the underlying metal cathode. Moreover, all these advantages are attained by a straightforward process readily adaptable to a variety of commercial applications.